This invention relates to a real-time data processor, and more particularly to a real-time azimuth correlator for a synthetic aperture radar (SAR) image processor.
Radar imaging using side-looking synthetic aperture radar techniques is the best known approach for achieving high-resolution imagery through planetary atmospheric cloud cover. However, if the radar echo data are not processed into images onboard the spacecraft or aircraft, very large quantities of raw uncorrelated data must be sent to Earth for processing. Conversely, if images are produced onboard in real time, multiple-look data may be superimposed into single frames and conventional data-compression algorithms may be applied to significantly reduce the data volume and rates transmitted to Earth.
During recent years, considerable effort has been devoted to the development of a digital radar image-processing capability. Unfortunately, results to date indicate that the digital data processing required to produce correlated radar images onboard a spacecraft or small aircraft is prohibitive based upon cost, complexity, power, size and weight considerations. Since only limited data reduction, by means of presumming and time expansion, can be accomplished with the uncorrelated radar echo data, proposed radar mission requirements to date have implied the need for reliable high-speed and high-capacity tape recorders for storage and have imposed potentially severe requirements upon the telecommunications link and ground data-handling capabilities.
Recent development of charge-coupled device (CCD) technology offers the potential for considerable simplification of the complicated digital implementation of SAR convolution. For instance, a CCD transversal filter of length N provides N stages of storage while performing N signal-by-weighting coefficient multiplications each clock period. Since the powerful computational equivalency of a CCD transversal filter significantly alleviates the normal constraints associated with digital processing, a CCD SAR image processor offers a potentially attractive solution to the real-time onboard SAR signal-processing problem, as described by the inventor in a U.S. Pat. No. 4,045,795 titled "Charge-Coupled Device Data Processor for an Airborne Imaging System."
It is well recognized that azimuthal resolution in radar imaging is proportional to the size of the antenna. The physical size of a real aperture antenna normally becomes too large to achieve an azimuth resolution comparable to the range resolution available from typical radar band-widths. The process wherein a small antenna is used to simulate a large antenna in order to achieve a practical azimuth resolution is termed synthetic aperture. Synthetic aperture radar (SAR) is based upon the fact that there is no difference between a large aperture antenna and a small antenna that successively occupies all of the positions which are simultaneously occupied by the larger antenna, provided the data are successively collected, stored, and subsequently combined to simulate the larger antenna.
The problem in SAR data processing is thus collecting and correlating the echo-return pulses in N range bins, where N is a function of the desired range resolution. A resulting set of N range-line samples for a given azimuth position is called a range line. For each subsequent transmitted pulse, a new range line is generated. Since the radar physically moves in the time interval between transmitted pulses, each range line will be at a different azimuthal position. A number of range lines corresponding to the number of echo returns required to synthetically simulate the desired real aperture antenna must be stored. The resulting matrix consists of rows that contain different time delays or ranges, and columns that provide azimuthal information for a given range. The information required to produce a single image from the matrix is dispersed throughout the matrix. In the time domain, correlation must be accomplished in both the range and azimuth dimensions in order to convert the dispersed echo data into image elements.
The primary processing functions employed to convert echoes into image elements are sampling, presumming, range correlation and azimuth correlation. The echo returns are sampled at greater than Nyquist rate and stored. The stored samples are then read out at a lower rate over the full pulse repetition interval (PRI), thus resulting in a time expansion of N samples in the PRI, i.e. a data-rate reduction proportional to the echo pulse duty cycle.
For SAR applications, the PRF is such that the azimuth resolution frequently exceeds the range resolution. On the assumption that the azimuth resolution does not have to be greater than that in range, echo pulses may be presummed (range bin 1 added to range bin 1, range bin 2 added to range bin 2, etc., over successive echo pulses). If the azimuth resolution were 6.25 meters, then presumming every four pulses into one composite pulse would provide a resolution of 25 meters. The result would be a data-rate reduction of 4. It should be noted that the presumming function is not practically accomplished by direct summation, but must be achieved by means of filtering to adequately reduce aliasing effects.
Range correlation of an incoming echo signal from a given target with a replica of the transmitted signal results in a compressed pulse having a pulse width corresponding to the range resolution and a position in range corresponding to that of the actual target. The correlated signal pulse width is inversely proportional to the transmitted signal bandwidth. Large bandwidths yielding high resolution can be accommodated because pulse compression (correlation) techniques allow the signal bandwidth to be expanded with negligible sensitivity loss.
The primary problem of real-time SAR data processing is with azimuth correlation. Signals from a given target will be received during transit of the SAR through the desired real aperture. Due to the Doppler effect, the carrier return from the target will be frequency modulated due to the SAR motion through the desired real aperture. This FM is treated as a chirp function and is assumed to be a part of the input signal to the azimuth filter corresponding to the range bin in which the designated target lies. Correlation of this signal with the expected Doppler chirp function across all azimuthal target positions relative to the SAR in the desired real aperture yields a compressed pulse having a pulse width corresponding to the azimuth resolution and a position in azimuth corresponding to that of the actual target. More correlation points (i.e., a longer correlation time) simulates a larger real aperture and therefore provides a narrower pulse width and improved azimuth resolution.
Throughout the image-processing algorithm, it is desirable to measure both amplitude and phase. This is best accomplished by I and Q processing wherein the vector of each echo return is resolved into real (I) and quadrature (Q) components such that the sum I.sup.2 + Q.sup.2 is proportional to the power of the echo return. In order to combine successive echo returns, as required for azimuthal processing, they must be resolved into their real (I) and quadrature (Q) components. It is the azimuth correlation process required for the I and Q signals that gives use to the problem solved by the present invention. In most spacecraft and some aircraft applications, azimuth correlation of both the I and Q signals with the Doppler reference function, requires that both a linear and quadratic range migration compensation capability be provided. Furthermore, a capability for reprogramming and updating both the range migration correction and Doppler reference functions in real-time must be provided to compensate for variations in vehicle flight parameters.